The crystallographic texture of a plate or sheet plays an important role in many applications. Crystallographic texture is crucial for the performance of the sputtering targets used to deposit thin films, due to the dependence of the sputtering rate on crystallographic texture.
The uniformity of thin films deposited from a sputtering target with non-uniform crystallographic texture is not satisfactory. Only a plate with uniform texture throughout its volume will give optimum performance.
The rate of sputtering from a grain in the target depends on the orientation of the crystal planes of that grain relative to the surface (ref. Zhang et al, Effect of Grain Orientation on Tantalum Magnetron Sputtering Yield, J. Vac. Sci. Technol. A 24(4), July/August 2006); the sputtering rate of each orientation relative to the plate normal is different. Also, certain crystallographic directions are preferred directions of flight of the sputtered atoms (ref. Wickersham et al, Measurement of Angular Emission Trajectories for Magnetron-Sputtered Tantalum, J. Electronic Mat., Vol 34, No 12, 2005). The grains of a sputtering target are so small (typically 50-100μm diameter) that the orientation of any individual grain has no significant effect. However, over a larger area (an area roughly 5 cm to 10 cm diameter) texture can have a significant effect. Thus, if the texture of one area on the surface of a target is different from the texture of any other area, the thickness of the film produced is unlikely to be uniform over the whole substrate. Also, if the texture of a surface area is different from that of the same area at some depth into the target plate, the thickness of the film produced on a later substrate (after the target is used, or eroded, to that depth) is likely to be different from that produced on the first substrate.
So long as the texture of one area, then, is similar to that of any other, it is not important what that texture is. In other words, a target plate in which every grain has a 111 orientation parallel to the plate normal direction (ND) is no better and no worse than one in which every grain has a 100 orientation parallel to ND, or than one which consists of a mix of 100, 111 and other grains, so long as the proportions of the mix remain constant from area to area.
Uniformity of film thickness is of major importance. In integrated circuits, several hundred of which are created simultaneously on a silicon wafer, for example, too thin a film at one point will not provide an adequate diffusion barrier, and too thick a film at another point will block a via or trench, or, if in an area from which it should be removed in a later step, will not be removable. If the thickness of the film deposited is not within the range specified by the designer, the device will not be fit for service, and the total cost of manufacture up to the point of test is lost, since no repair or rework is normally possible.
If the target does not have uniform texture, and thus does not provide a predictable, uniform sputtering rate, it is impossible, with state-of-the-art sputtering equipment, to control the variation of thickness from one point on the substrate to another. Partial, but not total, control of variation of thickness from substrate to substrate, and from target to target, is possible using test-pieces. Use of test-pieces, however, is time-consuming and costly.
With targets made according to the prior art, the non-uniformity of texture found in the target plate causes unpredictability or variability in the sputtering rate (defined as the average number of tantalum atoms sputtered off the target per impinging argon ion), leading to variations in the thickness of the film produced on a particular substrate, and also variations in film thickness from substrate to substrate and target to target.
Crystallographic texture also affects the mechanical behavior of a material. This is due to differences in the mechanical behavior of a single crystal of an anisotropic material when tested in different directions. Although single crystal materials are used in various applications, the majority of materials used in practice are polycrystals, which consist of many grains. If the grains forming a polycrystal have a preferred orientation (i.e. crystallographic texture), the material tends to behave like a single crystal having similar orientation. The formability of a material depends on the mechanical behavior of the material, which is a strong function of crystallographic texture.
Other material properties such as magnetic permeability are also influenced by crystallographic texture. For example, crystallographic texture is an important factor for the performance of a grain-oriented silicon steel, which is mainly used as the iron core for transformers and other electric machines. Improved magnetic properties, such as high magnetic permeability of the grain-oriented silicon steels, result in energy savings. To achieve good magnetic properties, a grain-oriented silicon steel should have strong <110>//ND and <100>//RD (rolling direction) texture (Goss orientation), which can then be easily magnetized in the rolling direction.
Crystallographic texture develops as a material is plastically deformed, and plastic deformation can only occur along certain slip systems that become active during deformation. Normal and shear strain components, along with other parameters such as temperature, determine which slip systems become active. Activation of a slip system causes grains to rotate towards a certain orientation, resulting in a crystallographic texture. The final crystallographic texture of a material is a strong function of both the starting texture and the strain induced in the material.
For example, during rolling of a plate in plane strain condition, material through the thickness of the plate is subjected to shear and normal strains simultaneously. The amount of shear strain varies significantly through the thickness of a plate. The mid-thickness of a plate is not subjected to any shear strain due to the symmetry of a conventional rolling process, whereas locations away from mid-thickness experience both shear and normal strains. Therefore, texture at the mid-thickness of a plate is considerably different than other locations.
Non-uniformity of texture through the thickness of a plate is referred to as the “through-thickness texture gradient”. Conventional rolling produces a plate or sheet with a strong through-thickness texture gradient. Neither the through-thickness texture gradient nor the main components of texture can be altered significantly by parameters which are varied and controlled in conventional rolling, such as % reduction in thickness per pass and rotation between passes.
Certain texture components, i.e. “rolling texture” components, become dominant in conventional rolling. Rolling texture components for a bcc metal are different than “shear texture” components, which form when a bcc metal is subjected to shear strain. When subjected to shear strain, the grains in a bcc metal rotate towards <110>//ND. An almost opposite behavior is observed for a fcc metal, which, when subjected to shear strain, will cause <111>//ND and <100>//ND to become the major texture components. The greater the shear strain introduced in a workpiece, the stronger the shear texture developed.
In a material (fcc or bcc) with a perfectly random texture, 10.2% of the volume (and 10.2% by number of the grains) has a <100> axis within 15-deg of ND. Another 13.6% of the volume has a <111> axis within 15-deg of ND and a further 20.4% of the volume has a <110> axis within 15-deg of ND. Therefore, a fcc material is said to have a shear texture if more than 10.2% of the volume has a <100> axis within 15-deg of ND, and more than 13.8% of the volume has a <111> axis within 15-deg of ND. A bcc material is said to have shear texture if more than 20.4% of the volume has a <110> axis within 15-deg of ND.
A higher plastic strain ratio (r-value) is known to enhance formability of a metal, and a bcc or fcc metal with a dominant <111>//ND texture component has higher plastic strain ratio (r-value). Therefore, shear texture with <111>//ND as one of the major components is desirable for improving the formability of a fcc metal.
The amount of shear strain through the thickness of a plate or sheet can be altered by switching from a conventional (symmetric) rolling to an asymmetric rolling process. The total amount of shear strain through the thickness can be increased, and more specifically, the mid-thickness can be subjected to some amount of shear strain, which is not possible in conventional rolling. Prior art asymmetric rolling methods include use of rolls with different diameters, rolls with different rotational speeds, and rolls with different surface properties that result in different friction coefficient between the top surface of a workpiece and the top roll, and the bottom surface of a workpiece and the bottom roll. Due to the difficulties in controlling the friction coefficient consistently, asymmetric rolling with different friction coefficients top and bottom is impractical and is excluded from further discussion here. These prior art methods can also be used to decrease the through-thickness texture gradient.
The application of the above-mentioned types of asymmetric rolling for introducing shear texture and minimizing texture gradient have been described in the prior art. See, e.g., Field et al., Microstructural Development in Asymmetric Processing of Tantalum Plate, J. Electronic Mat., Vol 34, No 12, 2005; Sha et al., Improvement of recrystallization texture and magnetic property in non-oriented silicon steel by asymmetric rolling, J. Magnetism and Magnetic Mat., Vol 320, 2008; Lee and Lee, Analysis of deformation textures of asymmetrically rolled steel sheets, Internat. J. Mech. Sci., Vol 43, 2001; Lee and Lee, Texture control and grain refinement of AA1050 Al alloy sheets by asymmetric rolling, Internat. J. Mech. Sci., Vol 50, 2008; Jin et al. Evolution of texture in AA6111 Al alloy after asymmetric rolling with various velocity ratios between top and bottom rolls, Mat. Sci. and Eng., Vol 465, 2007; Jin et al. The reduction of planar anisotropy by texture modification through asymmetric rolling and annealing in AA5754, Mat. Sci. and Eng., Vol 399, 2005; Kim et al. Formation of textures and microstructures in asymmetrically cold rolled and subsequently annealed aluminum alloy 1100 sheets, J. Mat. Sci., 2003; Zhang et al. Experimental and simulation textures in an as symmetrically rolled zinc alloy sheet, Scripta Materialia, Vol 50, 2004; and Kim et al. Texture and microstructure changes in asymmetrically hot rolled AZ31 magnesium alloy sheets, Mat. Lett. 59, 2005.
The asymmetric rolling methods described above introduce some amount of shear strain through the thickness of the plate by using asymmetry in the top and bottom roll diameter or the top and bottom roll speed. As the roll diameter or roll speed ratios of the top and bottom rolls increase, the shear strain introduced in the plate increases, but there are practical limits to these ratios and the amount of shear strain that can be introduced with these methods.